The relevance of lightweight materials has increased in the fields of automotive, railway or aviation technology as well in medical applications. Devices, which are a combination of metal or metal alloy and plastic workpieces, are often used in order to significantly reduce weight while maintaining desired properties, such as stiffness, corrosion resistance, impermeability or chemical resistance. In particular fiber reinforced polymers (FRP) are used in combination with lightweight metals to provide materials with excellent mechanical properties to industry. In this regard it is the challenge for technical development to provide reliable joining techniques, in particular in order to join metal components with plastic components.
From the prior art it is known that metal/plastic hybrid structures can simply be achieved by mechanical fastening. This method normally allows for easy disassembly, inspection and recycling of the components. The mechanical engagement can be accomplished by bolts, screws, clamps or rivets. Nevertheless, these mechanical connectors involve further elements which increase the weight of the entire workpiece. In particular for applications in aviation technology this is disadvantageous, because minimizing the weight of each device is a major concern in this field. A further disadvantage of mechanical fastening of hybrid structures is the fact that high stress concentration levels are generated at the location of the connection, and that the connection itself may be a starting point for a crack in at least one of the components. In particular reinforced polymers can significantly be altered in their properties by introducing the necessary holes for mechanical connections. Especially, the in-plane strength of the reinforced polymer can be strongly decreased. Furthermore, mechanical connections require the use of sealants due to hermetic losses.
Another preferred method is to join metal/plastic hybrid structures by adhesive bonding. This method consists in applying an adhesive between metal and plastic partners and an external pressure and/or heat during curing of the adhesive. This method requires intensive surface preparation of the materials associated with multi-procedural steps and a long curing time, making this process quite complex, expensive and time-consuming. Further it is often questionable whether a joint formed by an adhesive only is sufficiently stable. This is why this method is often combined with mechanical fastening.
Whereas in case of connections between two metal components conventional welding techniques such as fusion-based or friction-based welding have proven to result in stable joints even in situations where only punctual connections are possible. However, these techniques cannot simply be employed in the case of metal/plastic connections. Usually the welding temperatures for metals are much higher than for thermoplastic material, and thermoset materials cannot be welded at all since these materials do not melt. Often a prior surface treatment is required which is time consuming. Moreover, welding is often connected with a high energy input, which may lead to the problem that the material in the vicinity of the welding point is significantly influenced and the material properties are altered.
Another newly developed method is friction spot joining, described in U.S. 2011/0131784 A1, in which frictional heat is used to plasticize the metal partner and to subsequently melt the plastic workpiece to form a punctual joining. But this method is suitable only for low elasticity modulus alloys such as magnesium and aluminum, so that this technique cannot be used for joints where the metal part is made for instance of stainless steel or titanium alloys, which require much more heat to plasticize the metal part, inducing excessive thermal degradation of the polymeric workpiece.
In newly developed approaches to largely increase mechanical performance of metal/plastic overlap joints, such as structural stability and out-of-plane mechanical properties, 3D reinforcements are produced on the metal surface. It has been shown that these structures can be produced by treating the metal surface by electron beam technology, which is used to locally melt the metal surface to create holes and protrusions and subsequently the polymer (or composite) is added layer by layer. However, the electron beam technology requires a vacuum environment, restricting the size of the metal workpiece to be treated. Furthermore, the required deposition or lamination process makes this technology extremely time consuming. Another disadvantage of this technology is related to the reduced reproducibility of the individual pin geometries, limited by the unsupported build up of the molten metal when forming the pins. Another option is to use electric-arc welding to weld micro-pins on the metallic partner surface. However, all fusion-based welding process problems related with solidification cracks, hydrogen embrittlement, and evaporation of alloying elements also affect mechanical properties here.
Therefore, it would be desirable to join damage-tolerant and crash-resistant surface-structured and plastic workpieces, in particular for forming a lap joint, in a manner that can easily be applied and which does not have one or more of the afore-mentioned drawbacks.